JP2012072427A - Case hardened steel and method for manufacturing the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a case hardened steel which has favorable cold forgeability and superior impact properties after case hardening processing, and to provide an effective method for manufacturing the case hardened steel.SOLUTION: The case hardened steel contains C, Si, Mn, S, Cr, Al, Ti, Nb, B, and N, the remainder being iron and unavoidable impurities. Precipitates that are at least 20 μmamong precipitates containing Ti and/or Nb have a number density of not greater than 1.0 precipitate/mm, precipitates containing Mn and S that are over 5 μmand less than 20 μmamong the precipitates containing Ti and/or Nb have a number density of more than 0.7 but no greater than 3.0 precipitates per mm, and the fraction of ferrite is over 77 area%.

Description

  TECHNICAL FIELD The present invention relates to a case hardening steel that is a material of a machine part used by case hardening in transport equipment such as automobiles, construction machines, and other industrial machines, and a method for manufacturing the same. (Spindle gears, etc.), shafts, bearings, case hardening steels exhibiting excellent impact characteristics and excellent cold forgeability when subjected to case hardening for CVT pulleys, and methods for producing the same It is.

  Among machine parts used for automobiles, construction machines, and other various industrial machines, parts that require particularly high strength have been conventionally subjected to surface hardening heat treatment (skin-burning treatment) such as carburizing, carbonitriding, and nitriding. It has been broken. For these applications, case-hardened steels defined by JIS standards such as SCr, SCM, SNCM, etc. are usually used. After forming into a desired part shape by machining such as cutting or forging, surface hardening as described above. After being subjected to heat treatment, it is manufactured into parts through a finishing process such as polishing.

In recent years, it has been desired to reduce the manufacturing cost, lead time, and CO ₂ emission during manufacturing for the above machine parts. It is being changed to hot forging, and good cold forgeability is required. Moreover, in case hardening steel defined by JIS standard, since grain coarsening occurs by surface hardening heat treatment after cold forging, it is also important to suppress grain coarsening. Conventionally, by adding elements such as Al, Nb, and Ti to improve the problem of grain coarsening, precipitates such as AlN, Nb (CN), and TiC are finely dispersed. Techniques for stopping the movement of grain boundaries are used (for example, Patent Documents 1 to 8).

  In each of Patent Documents 1 to 8, by controlling the number of precipitates (carbides, carbonitrides, etc.) containing Nb and / or Ti having a predetermined particle size and composition within a predetermined range, It discloses that coarsening can be prevented, and although the effect of preventing crystal grain coarsening is obtained to some extent, the cold forgeability is not sufficient.

Japanese Patent Laid-Open No. 2007-217771 JP 2006-307271 A JP 2006-307270 A JP 2007-321211 A JP 2004-183064 A JP-A-11-335777 JP 2006-161142 A JP 2007-162128 A

  The present invention has been made in view of the circumstances as described above, and its purpose is to ensure the same crystal grain coarsening prevention property as that of the prior art, and to have good cold forgeability and the above machine. An object of the present invention is to provide a case-hardened steel excellent in impact characteristics after case-hardening treatment, which is usually required for parts, and to provide a useful method for producing the case-hardened steel.

The case-hardened steel of the present invention that has achieved the above-mentioned problems is C: 0.05 to 0.3% (meaning mass%, hereinafter the same for chemical composition), Si: 0.6% or less (including 0%) Mn: 0.20 to 1.0%, S: 0.001 to 0.025%, Cr: 1 to 2.5%, Al: 0.01 to 0.10%, Ti: 0.01 -0.10%, Nb: 0.01-0.10%, B: 0.0005-0.005%, N: 0.002-0.02% is satisfied, the balance is iron and inevitable impurities, Of the precipitates containing Ti and / or Nb, the precipitates of 20 μm ² or more have a number density of 1.0 piece / mm ² or less, and more than 5 μm ^{2 of the} precipitates containing Ti and / or Nb, be less than 20 [mu] m ^2, precipitates containing Mn and S, the number density of 0.7 pieces / mm ^2, greater than 3.0 pieces / mm ² or less There, it is characterized in that the ferrite fraction is 77 area percent.

  The case-hardened steel according to the present invention has (a) Mo: 2% or less (not including 0%), (b) Cu: 0.1% or less (not including 0%) and / or Ni as necessary. : It is also preferable to contain 0.3% or less (excluding 0%), and the characteristics of the case hardening steel are further improved according to the kind of element to be contained.

The present invention also includes a method for producing the case-hardened steel, and the production method of the present invention is a steel having any one of the above chemical compositions with a cooling rate from 1500 ° C to 800 ° C of 2.5 ° C / min or more. And after the first hot rolling at a rolling temperature of 970 to 1150 ° C., cooling to Ac ₃ point to 950 ° C., and further rolling temperature Ac ₃ point to The second hot rolling is performed at 950 ° C.

  According to the present invention, the chemical composition of steel is adjusted to a predetermined range, and the form (size) and number of composite precipitates containing Ti and / or Nb and containing Mn and S are set. Because it is prepared within the specified range, it can achieve good cold forgeability while ensuring the same level of grain coarsening prevention properties as before, as well as excellent impact properties after surface hardening heat treatment. can do. Therefore, the case-hardened steel of the present invention is useful as a material for various machine parts. Moreover, if the case-hardened steel of the present invention is used, the part forming by cutting can be replaced with cold forging, and lead time reduction and cost reduction of part forming can be achieved.

It is the schematic which shows the shape of the test piece of the cold forgeability measurement in the Example mentioned later. It is a graph which shows the heat processing conditions of the spheroidization process in the Example mentioned later. It is the schematic showing the shape of the Charpy impact test piece used for the measurement of an impact characteristic in the Example mentioned later. It is a graph which shows the carburizing process conditions in the Example mentioned later.

  In order to improve the cold forgeability of the case-hardened steel and to ensure the impact characteristics after the surface hardening heat treatment, the present inventors particularly set the chemical composition of the steel and the presence form (size, number, etc.) of precipitates. We focused on and examined it. As a result, the content of each component of C, Si, Mn, S, Cr, Al, Ti, Nb, B, and N is appropriately controlled, and is a precipitate containing Ti and / or Nb, If the form (size) and number density of the composite precipitate containing S and S (hereinafter referred to as “(Ti, Nb) -based composite precipitate”) and the number density are adjusted within a predetermined range, It has been found that cold forgeability superior to that of the prior art can be realized while securing the coarsening prevention characteristics, and further, impact characteristics after surface hardening heat treatment can be secured, and the present invention has been completed.

  Hereinafter, the chemical components of the case hardening steel of the present invention will be described.

C: 0.05-0.3%
C is an important element for securing the core hardness necessary for a component. If it is less than 0.05%, the static strength as a component is insufficient due to insufficient hardness. On the other hand, when the amount of C is excessive, the hardness becomes excessively high, and forgeability and machinability are reduced. Therefore, the C content is determined to be 0.05% or more and 0.3% or less. The amount of C is preferably 0.10% or more, more preferably 0.15% or more. Further, the C amount is preferably 0.27% or less, more preferably 0.25% or less.

Si: 0.6% or less (excluding 0%)
Si is an element effective in suppressing the decrease in hardness during the tempering process and ensuring the surface hardness of the part after skin tempering. Since such an effect increases as the content thereof increases, the Si content is preferably 0.01% or more, and more preferably 0.05% or more. On the other hand, when the amount of Si is too large, the deformation resistance of the material increases and the forgeability deteriorates. Therefore, the Si amount is set to 0.6% or less. The amount of Si is preferably 0.55% or less, more preferably 0.50% or less.

Mn: 0.20 to 1.0%
Mn acts as a deoxidizer, acts to increase the internal quality of the steel by reducing oxide inclusions, and has the effect of significantly enhancing the hardenability during case hardening such as carburizing and quenching. In addition, coarse particles containing Nb and / or Ti are formed by complex precipitation with carbides, nitrides, and carbonitrides (hereinafter referred to as “carbides”) containing Nb and / or Ti. It is possible to suppress the deterioration of cold forgeability due to the carbides and the like. Furthermore, if the amount of Mn is small, red heat embrittlement occurs and productivity decreases. Therefore, the amount of Mn is set to 0.20% or more. The amount of Mn is preferably 0.30% or more, and more preferably 0.35% or more. On the other hand, when the amount of Mn is excessive, there are adverse effects such as an increase in deformation resistance during cold forging and a marked segregation of stripes, resulting in increased material variation. Therefore, the amount of Mn is set to 1.0% or less. The amount of Mn is preferably 0.85% or less, more preferably 0.80% or less.

S: 0.001 to 0.025%
S is an element necessary for bonding with Mn, Ti, etc. to form MnS, TiS, etc., and forming a composite precipitate containing Mn and Ti. On the other hand, if the amount of S is excessive, the impact characteristics are adversely affected. Therefore, the S amount is determined to be 0.001 to 0.025%. The amount of S is preferably 0.005% or more, more preferably 0.010% or more. Further, the S amount is preferably 0.022% or less, more preferably 0.020% or less.

Cr: 1 to 2.5%
Cr is an element necessary for obtaining an effective hardened layer during case hardening such as carburizing. On the other hand, when the amount of Cr becomes excessive, excessive carburization is caused, which adversely affects the sliding characteristics of the parts after case hardening. Therefore, the Cr content is determined to be 1 to 2.5%. The amount of Cr is preferably 1.2% or more, and more preferably 1.3% or more. Further, the Cr amount is preferably 2.2% or less, more preferably 2.0% or less (more preferably 1.9% or less).

Al: 0.01-0.10%
Al is an element effective to combine with N to produce AlN and suppress the grain growth of the steel during heat treatment. Moreover, by adding together with Ti and Nb, which will be described later, AlN is combined with precipitates containing Ti and Nb, and the effect of preventing grain coarsening is more stable than when single precipitation is performed. On the other hand, when the amount of Al becomes excessive, the amount of solute Al increases, leading to an increase in deformation resistance during cold forging. Therefore, the Al amount is determined to be 0.01 to 0.10%. The amount of Al is preferably 0.02% or more, and more preferably 0.03% or more. Moreover, Al amount becomes like this. Preferably it is 0.09% or less, More preferably, it is 0.08% or less.

Ti: 0.01-0.10%
Ti produces | generates the carbide | carbonized_material etc. (Ti (C, N)) of fine Ti in steel, and has the effect which suppresses the crystal grain coarsening at the time of skin baking. On the other hand, when the amount of Ti is excessive, the production cost of the steel material is increased, and cold forgeability and impact characteristics (impact strength expressed by Charpy absorbed energy, etc.) are reduced due to the generation of coarse Ti-based inclusions. Therefore, the Ti amount is determined to be 0.01 to 0.10%. The amount of Ti is preferably 0.02% or more, and more preferably 0.03% or more. Further, the Ti amount is preferably 0.09% or less, and more preferably 0.08% or less.

Nb: 0.01 to 0.10%
Nb produces fine Nb carbides and the like (Nb (C, N)) in steel and has an effect of suppressing crystal grain coarsening during case baking. On the other hand, when the amount of Nb becomes excessive, the manufacturing cost of the steel material rises, and cold forgeability and impact characteristics (impact strength, etc.) decrease due to the generation of coarse Nb-based inclusions. Therefore, the Nb amount is determined to be 0.01 to 0.10%. The Nb amount is preferably 0.02% or more, and more preferably 0.03% or more. Further, the Nb amount is preferably 0.09% or less, more preferably 0.08% or less.

B: 0.0005 to 0.005%
B has the effect of significantly improving the hardenability of the steel material in a small amount, and also has the effect of strengthening the grain boundaries and increasing the impact strength. On the other hand, even if the amount of B becomes excessive, the effect is saturated and B nitride is easily generated, and cold workability and hot workability are deteriorated. Therefore, the B amount is determined to be 0.0005 to 0.005%. The amount of B is preferably 0.0007% or more, and more preferably 0.0010% or more. Further, the B amount is preferably 0.004% or less, and more preferably 0.0035% or less.

N: 0.002 to 0.02%
N is an element necessary for generating Ti or Nb and nitride, or carbonitride, but when the amount of N is excessive, Ti-based nitride is likely to be coarsened, resulting in a decrease in impact strength, This causes a decrease in cold forgeability due to an increase in deformation resistance. Therefore, the N amount is determined to be 0.002 to 0.02%. The amount of N is preferably 0.003% or more, and more preferably 0.005% or more. Further, the N amount is preferably 0.018% or less, and more preferably 0.015% or less.

  The basic components of the case hardening steel of the present invention are as described above, and the balance is substantially iron. However, it is naturally allowed that inevitable impurities brought into the steel depending on the situation of raw materials, materials, manufacturing equipment, etc. are contained in the steel. Furthermore, in this invention, in the range which does not inhibit the effect of this invention, you may contain the following arbitrary elements, and it becomes possible to further improve the characteristic of case hardening steel according to the kind of element to contain. .

Mo: 2% or less (excluding 0%)
Mo has an effect of remarkably improving the hardenability during case hardening such as carburizing and quenching, and is an element effective for improving the impact strength. Therefore, the Mo amount is preferably 0.01% or more, and more preferably 0.05% or more. On the other hand, when the amount of Mo becomes excessive, the machinability becomes poor because the hardness of the steel material increases. Therefore, the Mo amount is preferably 2% or less, more preferably 1.5% or less, and still more preferably 1.0% or less (particularly 0.8% or less).

Cu: 0.1% or less (not including 0%) and / or Ni: 0.3% or less (not including 0%)
Since both Cu and Ni are elements that are less likely to be oxidized than Fe, they are elements that improve the corrosion resistance of the steel material. Ni also has the effect of improving the impact resistance of the steel material. Therefore, both the Cu content and the Ni content are preferably 0.01% or more, and more preferably 0.05% or more. On the other hand, when the amount of Cu is excessive, the hot ductility of the steel material is lowered, and when the amount of Ni is excessive, the cost of the steel material is increased. Therefore, the Cu content is preferably 0.1% or less, more preferably 0.08% or less, and still more preferably 0.05% or less. The amount of Ni is preferably 0.3% or less, more preferably 0.2% or less, and still more preferably 0.1% or less. Cu and Ni may be added alone or in combination, but when Cu is added, it is preferable to add Ni.

  It is an object of the present invention to obtain crystal grain coarsening prevention characteristics equivalent to those of the prior art, to obtain higher cold forgeability than before, and to obtain excellent impact characteristics after surface hardening heat treatment. According to the study by the present inventors, it is considered necessary to suppress the coarsening of crystal grains in order to obtain excellent impact characteristics. In order to suppress the coarsening of the crystal grains, it is necessary to finely disperse the carbides of Ti and Nb. However, the carbides of Ti and Nb are not all fine, and coarse carbides and the like are also precipitated. Such coarse carbides are not preferred because they are harder than the matrix and adversely affect cold forgeability. Therefore, as a result of the study by the present inventors, even if it is a coarse carbide or the like, MnS and a composite precipitate such as a carbide of Ti and / or a carbide of Nb ((Ti, Nb) -based composite precipitate) and It has been found that the deterioration of cold forgeability can be suppressed by the action of MnS, which is softer than the matrix.

Specifically, 5 [mu] m ² than of the precipitates containing Ti and / or Nb, and less than 20 [mu] m ^2, the number density of precipitates containing Mn and S 0.7 pieces / mm ^2, greater than 3 0.0 piece / mm ² or less. In the present invention, 5 [mu] m ² than, 20 [mu] m ² less than the size of the (Ti, Nb) to that target based composite precipitates carbides of Ti and / or Nb is included in the complex precipitates in this size grain This is because the influence on both the coarsening prevention characteristics and the cold forgeability is large. That is, precipitates of 5 μm ² or less have little influence on cold forgeability, while precipitates having a size of 20 μm ² or more have a very large adverse effect on cold forgeability in the first place. Therefore, by improving the cold forgeability with precipitates having a size of more than 5 μm ^{2 and} less than 20 μm ² , it is possible to improve the cold forgeability while maintaining the effect of preventing grain coarsening. Although the precipitate containing Ti and / or Nb itself is hard, the deformability as a single precipitate is improved by complex precipitation of soft MnS to form a (Ti, Nb) composite precipitate. In addition, the effect of Ti and / or Nb carbides and the like can ensure crystal grain coarsening prevention characteristics during skin baking. Order to sufficiently exhibit the effect of improving cold forgeability and coarsening prevention properties, 5 [mu] m ² than of the precipitates containing Ti and / or Nb, and less than 20 [mu] m ^2, containing Mn and S The number density of precipitates is more than 0.7 / mm ² . The number density is preferably 1.0 piece / mm ² or more, more preferably 1.1 piece / mm ² or more, and further preferably 1.2 piece / mm ² or more. On the other hand, even if such a precipitate is deposited excessively, the strength after skin baking becomes insufficient. Therefore, the number density is 3.0 pieces / mm ² or less. The number density is preferably 2.5 pieces / mm ² or less, more preferably 2.0 pieces / mm ² or less. Further, 5 [mu] m ² than of the precipitates containing Ti and / or Nb, and less than 20 [mu] m ^2, the number density of those containing no Mn and S are generally at 1.0 to 10.0 cells / mm ² approximately is there.

Further, among precipitates containing Ti and / or Nb, precipitates having a size of 20 μm ² or more (the upper limit of the size of the precipitate is usually about 30 μm ² ) has a large adverse effect on cold forgeability, It is necessary to reduce the number as much as possible. Therefore, of the precipitates containing Ti and / or Nb, the precipitates of 20 μm ² or more have a number density of 1.0 pieces / mm ² or less. Of the precipitates containing Ti and / or Nb, the number density of the precipitates of 20 μm ² or more is preferably 0.9 pieces / mm ² or less, more preferably 0.8 pieces / mm ² or less. . In addition, as long as the component system of the present invention and the production method described later are used, the precipitates of 20 μm ² or more among the precipitates containing Ti and / or Nb usually do not contain Mn and S, but may contain these. There is no adverse effect and is within the scope of the present invention. The number of precipitates having a size of 20 μm ² or more can be adjusted by adjusting the amount of Ti and / or Nb added to the steel, or in the production method described later, the heating temperature, heating time, It can be controlled by adjusting the processing temperature or the like.

The number density of precipitates containing Ti and / or Nb and having a particle size of 5 μm ² or less (however, 2 μm ² or more as described in Examples below) usually contains (i) Mn and S. The composite precipitate is about 0.0 to 0.5 pieces / mm ² , and (ii) the precipitate containing no Mn and S is about 0.1 to 1.5 pieces / mm ² .

  The case-hardened steel of the present invention has a ferrite fraction exceeding 77 area%. This is because if the ferrite fraction is low, the cold forgeability is impaired. The ferrite fraction is preferably 80 area% or more, more preferably 82 area% or more, and still more preferably 83 area% or more. The remaining structure other than the ferrite structure is, for example, pearlite, bainite, martensite, or the like.

  In producing the case-hardened steel of the present invention, in a series of processes such as smelting, casting, soaking, split rolling, hot rolling, the cooling rate during casting is particularly fast, It is important to prevent the soaking temperature from becoming too high, and to appropriately control the respective temperature ranges in two stages of hot rolling. Detailed conditions for each step are as follows.

  In casting, it is important to finely disperse MnS that crystallizes during cooling. Specifically, the cooling rate from 1500 ° C. to 800 ° C. during casting is 2.5 ° C./min or more. In order to set the cooling rate to 2.5 ° C./min or more, for example, the amount of mist sprayed in the cooling zone during continuous casting may be increased more than usual. The cooling rate is preferably 2.8 ° C./min or more, more preferably 3.0 ° C./min or more.

  In the heating (soaking) before the block rolling, it is important not to make the MnS finely dispersed during cooling during the casting cool, and the heating (soaking) temperature is 1100 to 1200 ° C. And The heating temperature is preferably 1180 ° C. or lower, more preferably 1170 ° C. or lower. Moreover, after the partial rolling, it is preferably cooled to room temperature at 5 ° C./second or less, more preferably 3 ° C./second or less. The heating time is not particularly limited, but is, for example, about 0 to 100 minutes at a soaking temperature.

In hot rolling, it is important to perform rolling in two stages by changing the temperature range. In the first time, Ti and / or Nb carbides are complex-precipitated in MnS finely dispersed during casting, and in the second time, the ferrite content is reduced. Secure rate. Specifically, after hot rolling at a first processing temperature of 970 to 1150 ° C., cooling is performed to Ac ₃ point to 950 ° C., and the second time is hot rolling at a processing temperature of Ac ₃ point to 950 ° C. To do. The first processing temperature is preferably 1000 to 1130 ° C, more preferably 1020 to 1100 ° C. The processing temperature for the second time is preferably 800 to 930 ° C. The cooling rate from the first processing temperature to the second processing temperature is not particularly limited, but is about 10 ° C./second, for example. The cooling rate after the second rolling is preferably 5 ° C./second or less so that bainite and martensite are not generated.

  Hereinafter, the present invention will be described more specifically with reference to examples. The present invention is not limited by the following examples, and can of course be implemented with appropriate modifications within a range that can be adapted to the above-described gist. Included in the range.

  Steels having chemical components shown in Tables 1 to 3 are melted in accordance with a normal melting method, cast, soaked, and then subjected to hot forging (simulating the above-described partial rolling) and cooled to room temperature (cooling rate) Is 5 ° C./second). Then, after reheating and performing the first forging (simulating the first hot rolling described above) and cooling to the second forging temperature (simulating the second hot rolling described above), the second forging Forging was performed to cool to room temperature (cooling rate was 5 ° C./second) to obtain a steel bar having a diameter of 30 mm. Tables 1 to 3 show the cooling rate (° C / min), the soaking temperature (° C), the soaking time (min), the first and second forging temperatures (° C) during casting.

  The obtained steel bar was measured by the following method.

(1) Measurement of precipitates A longitudinal section (surface parallel to the axis) of the obtained steel bar at the D / 4 position (D is the diameter of the steel bar) is polished and measured by an automatic EPMA in an arbitrary range of 10 mm × 10 mm. Went. For inclusions of 2 μm ² or more, a case where the Ti content is 5% by mass or more is judged as “contains Ti”, and a case where the Nb content is 5% by mass or more is judged as “contains Nb” did. In addition, regarding Mn and S, when the content was 5% by mass or more, it was determined that “contains Mn” and “contains S”, respectively. Detailed measurement conditions are as follows.
EPMA analyzer: JXA-8100 type electronic microprobe analyzer (manufactured by NEC Corporation)
Analyzer (EDS): SystemSix (manufactured by Thermo Fisher Scientific)
Acceleration voltage: 15 kV
Operating current: 4nA
Observation magnification: 200 times

(2) Measurement of cold forgeability As shown in FIG. 1, a test piece of φ20 mm × 30 mm was cut out from the obtained steel bar, and the test piece was spheroidized as shown in FIG. 2, that is, heated to 740 ° C. The temperature was maintained for 4 hours, cooled to 650 ° C. at a cooling rate of 5 ° C./hour, and subjected to a furnace heat treatment from 650 ° C. to room temperature. The test piece subjected to the spheroidization treatment was subjected to an end face constrained compression test at a rolling reduction of 50%, and the deformation resistance value (N / mm ² ) was measured.

(3) Measurement of impact characteristics A test piece having the shape shown in FIG. 3 was collected from the obtained steel bar, and the test piece was subjected to carburizing conditions shown in FIG. 4 (the carburizing period conditions were temperature: 950 ° C., time: 100 minutes, Carbon potential: 0.8%, carburizing gas: propane, diffusion phase conditions: temperature: 850 ° C, time: 60 minutes, carbon potential: 0.8%, carburizing gas: propane, quenching conditions are oil-cooled to 80 ° C )), Followed by tempering at 160 ° C. for 180 minutes and air cooling. About the test piece after the said tempering, the Charpy impact test was performed at normal temperature according to JISZ2242, and the Charpy impact value (J / cm < ² >) was measured.

(4) Observation of structure The steel bar is embedded in the support substrate in a state where the longitudinal section (surface parallel to the axis) of the D / 4 position (D is the diameter of the steel bar) is exposed. After being immersed for 2 seconds and corroded, the range of 700 μm × 900 μm was observed and photographed with an optical microscope, and the tissue was identified and the area ratio was measured.

(5) Measurement of crystal grain size A cylindrical test piece of φ20 mm × 30 mm was taken from the steel bar, and the cylindrical test piece was compressed in the height direction at room temperature (compression rate: 85%, height: 3 mm), and then the above Carburization and tempering were performed under the same conditions as (3) (the conditions described in FIG. 4), and the crystal grain size was measured. The crystal grain size is measured by etching with the carburized layer at the location where the equivalent strain is 1.2 on the cross section of the test piece subjected to carburizing and tempering treatment as the speculum position, and then observing with an optical microscope (magnification: 200 times). The particle size number of the prior austenite grains was determined according to JIS G0551.

  The results are shown in Tables 4-6. Tables 4 to 6 also show the number of precipitates containing Ti and / or Nb other than those defined in the present invention.

No. 1 to 49, because the component composition and the manufacturing method are appropriately controlled, 5 [mu] m ² than, 20 [mu] m ² less than the size of the (Ti, Nb) based composite precipitates and 20 [mu] m ² or more (Ti, Nb) based precipitates Since the product satisfies the requirements of the present invention and the ferrite fraction is more than 77 area%, good cold forgeability and impact characteristics are realized. In addition, as shown in Tables 4-6, No. None of (Ti, Nb) -based precipitates of 20 μm ² or more in 1 to 49 contained Mn and S.

  On the other hand, no. Nos. 50 to 61 were insufficient in at least one of cold forgeability and impact characteristics because at least one of the component composition and the production method did not satisfy the requirements of the present invention.

No. 50, Mn and Al amount is large, and because the forging corresponding to the hot rolling was not performed only only second time conditions, 5 [mu] m ² than, 20 [mu] m ² less than (Ti, Nb) based composite precipitates and ferrite The fraction was insufficient and cold forgeability became insufficient.

No. 51, without forging first time, and because the forging temperature of the second time was high, 5 [mu] m ² than, 20 [mu] m ² less than (Ti, Nb) based composite precipitates and ferrite fraction is insufficient and also 20 [mu] m ² or more The (Ti, Nb) -based precipitates became excessive and cold forgeability became insufficient.

No. 52 has a high soaking temperature before forging corresponding to slabbing, but also because they did not take place, the single forging corresponding to hot rolling, 5 [mu] m ² than, 20 [mu] m ² less than (Ti, Nb) based composite Precipitates and ferrite fraction were insufficient, and cold forgeability became insufficient.

No. 53, since the Ti content is large and also was not performed, the single forging corresponding to hot rolling, 5 [mu] m ² than, 20 [mu] m ² less than (Ti, Nb) based composite precipitates and ferrite fraction is insufficient and The (Ti, Nb) -based precipitates of 20 μm ² or more became excessive, and the cold forgeability became insufficient.

No. 54, since the amount of Cr is large and also was not performed, the single forging corresponding to hot rolling, 5 [mu] m ² than, 20 [mu] m ² less than (Ti, Nb) based composite precipitates is insufficient, cold forgeability It became insufficient. No. 55, since the Nb content is large and also was not performed, the single forging corresponding to hot rolling, 5 [mu] m ² than, 20 [mu] m ² less than (Ti, Nb) based composite precipitates and ferrite fraction is insufficient, cold The forgeability and impact properties were insufficient.

No. 56 because it was not performed, the single forging corresponding to hot rolling, 5 [mu] m ² than, 20 [mu] m ² less than (Ti, Nb) based composite precipitates and ferrite fraction is insufficient, insufficient cold forgeability It became.

  No. No. 57 did not perform the first forging corresponding to hot rolling, so the ferrite fraction was insufficient and the impact characteristics were insufficient.

No. 58, the cooling rate during the casting is slow, slabbing soaking temperature before forging corresponding to rolling is high, and because you did not first-time forging corresponding to hot rolling, 5 [mu] m ² greater, less than 20 [mu] m ² The (Ti, Nb) -based composite precipitates were insufficient, and cold forgeability and impact properties were insufficient.

No. 59, because the soaking temperature before forging corresponding to slabbing was high, 5 [mu] m ² than, 20 [mu] m ² less than (Ti, Nb) based composite precipitates is insufficient, also 20 [mu] m ² or more (Ti, Nb ) System precipitates became excessive and cold forgeability became insufficient.

No. 60 and 61, higher soaking temperature before forging corresponding to slabbing, but also because they did not take place, the single forging corresponding to hot rolling, both 5 [mu] m ² greater, 20 [mu] m ² less than (Ti, Nb) system composite precipitate is insufficient. No. 61 further had an excess of (Ti, Nb) -based precipitates of 20 μm ² or more, so that cold forgeability was insufficient in all cases.

Claims (4)

C: 0.05-0.3% (meaning mass%, hereinafter the same for chemical composition)
Si: 0.6% or less (excluding 0%),
Mn: 0.20 to 1.0%
S: 0.001 to 0.025%,
Cr: 1 to 2.5%
Al: 0.01 to 0.10%,
Ti: 0.01-0.10%,
Nb: 0.01-0.10%,
B: 0.0005 to 0.005%,
N: 0.002 to 0.02%
The balance is iron and inevitable impurities,
Of the precipitates containing Ti and / or Nb, the precipitates of 20 μm ² or more have a number density of 1.0 piece / mm ² or less,
5 [mu] m ² than of the precipitates containing Ti and / or Nb, and less than 20 [mu] m ^2, precipitates containing Mn and S, the number density of 0.7 pieces / mm ^2, greater than 3.0 groups / mm ² or less,
A case-hardened steel having a ferrite fraction exceeding 77 area%.   Furthermore, the case hardening steel of Claim 1 containing Mo: 2% or less (it does not contain 0%).   The case hardening steel according to claim 1 or 2, further comprising Cu: 0.1% or less (not including 0%) and / or Ni: 0.3% or less (not including 0%). The steel having the chemical composition according to any one of claims 1 to 3,
Casting at a cooling rate from 1500 ° C to 800 ° C at 2.5 ° C / min or more,
Split rolling at a heating temperature of 1100 to 1200 ° C,
After the first hot rolling at a rolling temperature of 970 to 1150 ° C., cooling to Ac ₃ point to 950 ° C., and further performing the second hot rolling at a rolling temperature Ac ₃ point to 950 ° C. A method for producing case-hardened steel.

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